1. Field of the Invention
The present invention relates to a technology of transmitting radio frequency for wireless communication, and more particularly to a remote access unit for transmitting and receiving channels having different transmission schemes in a bi-direction.
2. Description of the Related Art
In recent wireless communication, various kinds of wireless communication services are provided. These services are referred to as 2G wireless communication service, 3G wireless communication service, wireless local area network (LAN), wireless Internet, etc. Additional wireless communication services use a radio-over-fiber type optical network such as an optical communication network combined with a wireless transmission system. The radio-over-fiber type optical network converts radio signals into optical signals and then transmits the radio signals as a form of the optical signals and converts the optical signals into radio signals to transmit the radio signals wirelessly.
FIG. 1A illustrates a schematic block diagram showing a conventional remote access unit. Referring to FIG. 1A, the conventional remote access unit 100 including a first amplifier 110 for amplifying downstream data in which time division channels (TDD) and frequency division channels (FDD) are multiplexed, a second amplifier 150 for amplifying upstream signals received wirelessly, an antenna 130 for receiving a upstream data wirelessly and transmitting the downstream data wirelessly, a circulator 120 for providing the downstream data to the antenna 130 and also providing the upstream data received from the antenna toward the second amplifier 150, and a filter 140 located between the circulator 120 and the second amplifier 150.
FIG. 1B illustrates an exemplary wave form of upstream data in which upstream time division channels, upstream frequency division channel and frequency division channels of the downstream data are multiplexed. FIG. 1B illustrates a state in which some of the downstream frequency division channels are removed from the upstream data by the filter 140.
FIG. 2 illustrates an exemplary schematic block diagram showing a structure of an optical network including the conventional remote access unit shown in FIG. 1A. Referring to FIG. 2, the conventional optical network 200 includes a central station 210 and a remote access unit 220. The central station 210 includes an electric-optical converter 211 for converting downstream data into downstream optical signals, and a photo-electric converter 212 for converting upstream optical signals into upstream data.
The remote access unit 220 includes a photo-electric converter 221 for photo-electrically converting the downstream optical signals into downstream data, a first amplifier 222 for amplifying the downstream data, an electric-optical converter 226 for electric-optically converting the upstream data into upstream optical signals, and a second amplifier 225 for amplifying the upstream data. A circulator 223 is located between the first and second amplifiers 222 and 225, which is connected to an antenna 224.
Therefore, the conventional remote access unit and the optical network including the same can input and output signals in which time division channels and frequency division channels are multiplexed, but cannot separate and process channels from one another, based on transmitting schemes. Thus, a part of downstream signals is mixed with upstream signals, thereby causing a deterioration of elements for processing upstream signals.